We have used quantitative ligand binding studies to characterize the multiple types and subtypes of opioid and other peptide receptors in rat brain, bovine adrenal medulla, bovine posterior pituitary, and other tissues. We have demonstrated the presence of receptors for oxytocin and a V1 subtype of vesopressin receptor in porcine testis, leyding cells, tunica albuginea, epididymis and vas deferens. A new, novel type of vasopressin receptor has been demonstrated in procine seminal vesicle at concentrations in excess of those in renal medulla. Unlike the V1 receptor, these receptors are coupled to adenylate cyclase. Neurosecretosomes from bovine posterior pituitary are shown to contain high concentrations of opioid receptors, which are exclusively of the "Kappa" type: Mu, delta and benzomorphan sites are not present. At least two classes of sites of differing affinities (Kd approximately 1 nM, Kd approximately 1 MuM) are present. Receptors for ouabain, digoxin, and digitoxin in rat brain and cardiac muscle have been characterized, and also consist of two classes of sites, both of which are sensitive to K+. The effects of fatty acids on the binding of these cardiac glycosides and of opioid peptides has been studied systematically.